The inventive subject matter relates to electronic devices, more particularly, to apparatus and methods for protecting electronic devices.
Power semiconductor devices, such as insulated gate bipolar transistors (IGBTs), metal oxide semiconductor field effect transistors (MOSFETs), integrated gate commutated thyristors (IGCTs) and gate turn off thyristors (GTOs) can switch electrical currents in microseconds or even fractions of a microsecond. Such high speed current switching can result in large voltage transients due to the effect of the current rate of change (di/dt) on the power circuit stray inductances. These voltage spikes can damage the semiconductor device and peripheral ancillary components.
Surge transient suppressors are commonly used to protect semiconductor switches against the effects of high voltage spikes. Such transient suppressors include metal oxide varistors (MOVs), which generally are fast transient voltage suppression power devices, but typically exhibit a non-linear clamping voltage versus clamping current behavior. For example, in a relatively large MOV, the operating voltage may be around fifty percent of the clamping voltage at 200A.
A silicon IGBT may have an instantaneous voltage rating of 6.5 kV but, due to the effects of cosmic ray degradation a maximum continuous voltage rating of only 3.8 kV. Accordingly, operating the IGBT at its continuous voltage rating results in a voltage utilization ratio of around 3.8 kV/6.5 kV. To cover for circuit tolerances, one may choose to protect that IGBT with an MOV having a clamping voltage of 6 kV, which is 500V below the maximum instantaneous voltage rating of the IGBT. However, such an MOV may have a continuous operating voltage of only 3 kVdc, which means operating the IGBT at less than its rated continuous DC voltage to provide the desired clamping protection.
Voltage utilization ratio may be tighter in silicon carbide (SiC) power semiconductors. For example, a 10 kV SiC MOSFET or IGBT may be capable of continuous utilization at 8 kVdc, which would entail the use of an MOV having a continuous operating voltage of at least 8 kVdc. However, an MOV with that rating may have a clamping voltage of around 16 kV, which is significantly greater than the SiC semiconductor device's instantaneous voltage rating, and thus may provide inadequate protection. If an MOV with a maximum clamping voltage at around 9.5 kV is used, then the 10 kV SiC semiconductor will be protected but at the price of using a maximum continuous operating voltage of only 4.75 kV, which is well below the 8 kVdc capability of the SiC semiconductor device. Techniques for intermittent application of an MOV to a protected device to reduce steady state voltage across the MOV during normal operation are described in U.S. Pat. No. 5,621,599 to Larsen et al., but there is an ongoing need for improved overvoltage protection techniques.